Backing up a bit, though, here are the burning questions yet to be answered at this nascent stage in the proceedings:

WHERE WILL THE CROSSOVER TAKE PLACE?Supergirl‘s “National City” is an answer to the first question, but merely a partial one. Because as fans of The Flash, especially, know, there’s a multitude of universes (a “multiverse,” one might say) to choose from.

The Flash (and Arrow and Legends of Tomorrow, by association) take place on Earth-One, where to date there has been zero direct mention of Supergirl or her higher-profile cousin Superman. Yes, Legends‘ Rip Hunter alludes in his Feb. 4 monologue to “men of steel” (and “dark knights,” as Batman is sometimes dubbed), but neither Supes nor the Girl of Steel have been specifically name-checked. Plus, early episodes of The Flash made clear that superpowered beings were a new thing to Earth, and certainly not the stuff of regular Daily Planet headlines.

All of which is a long way of saying: Supergirl must not live in Barry’s world/universe. To that, some will retort, “Well, then she must be on Earth-Two.” To that, I counter: National City has not displayed even a whiff of the retro/art deco Earth-Two aesthetic we have glimpsed on The Flash. That leads me, at least, to rule out Earth-Two as Kara’s second home. Maybe another alternate Earth is Barry’s springtime stop? (As TVLine reader Roger noted, Earth-Three is home to Ultragirl instead of Supergirl.) Whatever the case, the episode title’s nod to worlds supports the general theory — as well as allows for another possibility, that each hero is plucked from their current universe and deposited on a common “away” field to battle some Super Bad.

HOW WILL THE CROSSOVER TAKE PLACE?
Working off the theory that Barry is bound for an alternate Earth, that is where the “breaches” on The Flash come into play. Simply said, breaches are portals between, for now, Earth-One and Earth-Two. Allowing them to also open a door to yet another universe is an easy enough leap — hampered only by the fact that The Flash‘s current arc has Barry, Dr. Harrison Wells et al determined to shut down all the breaches ASAP.

WILL THE REST OF THE FLASH CAST GET LEFT BEHIND?
Thus far, only Gustin has been announced as crossing over to Supergirl. But that could simply be a case of leading with your lead, as well as not letting slip too much about the storyline that is being kept under wraps. But if it is only Barry who finds himself in National City on March 28, that invites our next, bigger question….

WILL THIS CROSSOVER BE SELF-CONTAINED ON SUPERGIRL?
Or will the preceding Tuesday’s episode of The Flash tee it up? Again, the press release offers no indication that the crossover will extend to the CW series — and yet that seems awfully improbable. Somehow, the sequence of events that transport Barry to this other Earth need to be shown on-screen. But would Supergirl use up its whole Act I to do that? Would the CBS drama open this “event” episode with a prologue so heavily laden with exposition for those viewers who aren’t Flash-familiar? It seems unlikely, but if that is how they play it, I return to the previous question — of casting — and opine that other Flash cast would have to be on hand, in Central City, where Barry’s trip to National City starts. (It’s not like you’d open with The Flash, by his lonesome, speeding down a street and, what, “tripping” into a breach?)

Another sticky wicket, with regards to a Supergirl-only setting for the crossover: Will Barry return to Central City with knowledge of the Kryptonian hero’s existence? Thus making The Flash, and Arrow by association, Supergirl-savvy? It’d seem a huge secret to keep, and you just know Cisco will want pictures!

WHO/WHAT WILL BE THE VILLAIN OF THE PIECE?
“What Big Bad will demand the combined forces of the Girl of Steel and the Fastest Man Alive?” is the wrong question to be asking, seeing as The Flash’s presence would seem to be unplanned (versus solicited). Rather, he’ll likely turn out to be a much-needed helping hand. Smallville‘s Laura Vandervoort begins her recurring run as Indigo in late February — might the baddie sometimes known as Brainiac 8 amass enough havoc to challenge two heroes working in tandem? Or, would the writers go so far as to hint at Supergirl and Flash’s comic-book deaths in the “Crisis on Infinite Earths” series, by casting Anti-Monitor or another perpetrator of that canvas-shaking tragedy? UPDATE: Or, as TVLine reader Mike notes, if “Worlds Finest” is meant to evoke the DC Comics Superman/Batman team-up, maybe The Flash chases one of his adversaries to National City?

And lastly…

WILL THERE BE SINGING?
Benoist and Gustin are of course both Glee grads. Supergirl‘s Winn, Jeremy Jordan, has Broadway cred. So, what better way to decompress after a hard day of allying against a supervillain than some karaoke, as Barry Allen has done at least once before on The Flash? (And should Kara ever pay a visit to Barry’s world, Broadway vet Jesse L. Martin can get in on the action as well.)

What questions — or educated answers — do you have about the crossover?